How to Grow and Care for Dahlias

Dahlias are late-season bloomers. They bloom from mid-summer through the first frost and are available in a vast array of colors, patterns, sizes, and flower forms. Plant size ranges from compact border varieties to species that have plate-sized blossoms atop 6-foot plants. Despite this diversity, most dahlias grow on long, erect stems that give the blooms room to show off. Native to Mexico and Central America, dahlias boast over 20,000 cultivars and 30 species and are the prized darlings of plant breeders and florists alike. However, pet lovers beware, as this species of flower is toxic to both dogs and cats.

This plant is a native perennial only in tropical climates and is considered an annual in zones lower than 8. For this reason, Dahlias can be temperamental and fussy about their growing conditions, yet many of the newer cultivars are more reliable and easy-growing.

Dahlias grow from tubers and can be planted outdoors after the last frost when the soil has warmed. Yet, they are too tender to leave in the ground all winter long in most zones. When grown as annuals, you'll need to dig up your dahlias and store the tubers indoors for winter, and then replant them in the spring. The plants reach maturity and bloom about eight to ten weeks after planting.

Dahlia Care

Stunning dahlias are actually fairly easy to grow. Grown from tubers, not bulbs, you can start dahlias indoors in early spring, and then plant them outdoors once the danger of frost has passed. Or, you can wait until the soil has warmed in the spring, and then plant the tubers directly outdoors. Plant tubers two to six inches deep, depending on variety. The plants need good air circulation to thrive, so place smaller varieties about 12 inches apart and large cultivars up to three feet apart. Regardless of where you grow them—in the ground or in containers—plant tubers in an area that receives full sun, water them regularly, and fertilize them promptly when new growth appears.

Dahlias require the one-time task of pinching off the tops, once the tubers have sprouted and formed branches. Wait until three sets of branches appear, and then top the stem just above the highest set of branches. This pruning tactic encourages the plant to grow more branches, thus creating more blooms. Within months, you'll be treated to a garden full of stunning, colorful, and oversized blooms. Just be sure to deadhead faded blooms to keep the plant looking tidy and to encourage flowering.

Light

In order to produce abundant blooms, dahlias require full sun, preferably at least 6 to 8 hours a day. In climates more similar to their native growing zone (USDA zones 8 and up) this plant will benefit from shade midafternoon, when the sun is especially hot.

Soil

Dahlias prefer rich, loamy soil that contains plenty of organic matter and drains well. If your backyard soil tends towards a denser clay, add in sand, peat moss, or manure to loosen the soil texture for better drainage. Dahlias thrive in a neutral soil pH of around 6.5.

Water

Plant dahlia tubers in the spring, and then let nature take its course until sprouting occurs. Do not water dahlia tubers until green growth shows above the surface. They will not need water before their root system develops.

Once sprouted, water your dahlias once or twice a week. Make sure to water deeply, as large tubers may be planted six inches deep. If summer days are especially hot and dry, you may need to water more frequently and never let the soil dry out.

Temperature and Humidity

Timing is especially important when it comes to planting dahlias, as they'll struggle to establish in cold soil. Wait until the final spring frost has passed and the ground temperatures have reached 60 degrees Fahrenheit.

You can start tubers in containers indoors—perhaps in a garage or greenhouse—to get them off to a quick start. It's safe to plant them outdoors when all danger of frost is passed. If you plan to dig up tubers and store them indoors for winter, make sure your storage area has a little humidity so they don't shrivel up and dry out.

Fertilizer

Dahlias are heavy feeders—the more food they receive, the larger the plant will grow, and subsequently, the bigger (and more numerous) their flowers will be. Use a fertilizer with a high percentage of phosphorus (perhaps a 10-30-20 ratio) to promote blooming. For the amount, follow the product label instructions. Do not use one with a high percentage of nitrogen. Too much nitrogen creates lush foliage, but few blooms.

If you plan to dig up and store your tubers for winter, stop fertilizing your plants at the end of August. You don't want to encourage more growth late in the season as you prepare the tubers for dormancy.

Types of Dahlias

Dahlias come in many different patterns, textures, and colors—the types are innumerable. A few favorite varieties take on different shapes and flower patterns and include the following:

• The pink Dahlia ‘Pianella’ is considered a "cactus dahlia" with double-flowering blooms and long, rolled petals that make it look spiny (like a cactus).
• The Dahlia ‘Kelvin Floodlight’ is yellow in color and has broad flat petals that are slightly rolled at the tip. Classified as a "decorative informal," the petals of this flower are irregularly placed, giving the bloom its full look.
• Certain types of dahlias, like Dahlia ‘Magenta Star’ feature only one row of slightly overlapping petals, taking on a much different look than fuller varieties. This particular one is named for both its color and appearance.
• The striking red Dahlia ‘Moor Place’ is deemed a "pompom dahlia" with small double-flowering blooms. The petals on this variety are round and tightly rolled, giving it a pompom look.

Propagating Dahlias

You can propagate dahlias either from cuttings or from overwintered tubers. Propagating from cuttings requires waiting until your tubers sprout in the spring. However, you can get a jump start on the growing season by dividing your dahlia tubers and planting them in containers indoors, before outdoor temperatures warm up. Come summer, your plants will be fully mature and may flower earlier. Plus, dividing your tubers before planting yields more plants and, ultimately, more flowers.

Here's how to propagate dahlias from tubers:

1. Gather your overwintered dahlia tubers, a trowel, garden shears or pruners, potting soil with vermiculite, peat, large growing containers, and a 5-gallon bucket.
2. Mix the soil and peat in the bucket, and water until barely moist. Transfer the soil into several different growing containers. (It's especially important that tubers have moist soil while they develop a root system and begin to sprout.)
3. Examine a clump of tubers and identify those with eyes (if you can't find them, put the clump in a warm, moist area for several days until eyes begin to swell or sprout). Cut those tubers from the clump at the neck. Depending on the size of the clump, you can remove several tubers from each one.
4. Dig a hole in the soil of each container (2 to 3 inches deep for small tubers, and 6 to 7 inches deep for larger ones). Lay single tubers horizontally in the planting hole with the eye pointing upwards or plant tuber clumps upright and vertically, with at least one inch of soil covering last year's stem.
5. Allow the tubers to sprout in a sunny window, while making sure the soil never completely dries out. Once sprouted with three sets of branches, pinch off the top.
6. When the soil warms in late spring, plant dahlias in your garden 3 to 4 feet apart, for larger varieties, and allow 2 to 3 feet in between rows.

Here's how to propagate dahlias from cuttings:

1. Gather a sharp knife, alcohol pads, potting soil, rooting hormone powder, and a 4-inch pot.
   
2. Wait until your tubers sprout and grow at least 3 inches tall. Then, clean your knife with the alcohol pads. Allow it to dry. Next, make your cut just below the sprout and partially into the tuber.
3. Lay your cutting on a hard surface and trim away the lower leaves. Prepare your pots with potting soil, and then poke three or four small holes along the edge of the pot.
4. Dip the end of your cutting into the hormone powder and place it into the hole, backfilling it with soil. Repeat with other cuttings and holes.
5. Water the pot, allow it to drain, and then place it in a sunny window. Keep the soil moist.
6. In two to three weeks, your cuttings should root. Once they are growing well and temperatures have warmed, plant them outdoors in your garden bed using the recommended spacing.

How to Grow Dahlias From Seed

Dahlias can be grown from seeds purchased at your local nursery. or from seeds collected from last year's plants. To do so, first, fill a seeding tray with seed starting mix and sow seeds indoors, directly into this medium, four to five weeks before the last frost. Move the tray to a sunny window and keep the soil moist. Once sprouted, allow the seedlings to form one true set of leaves before transplanting each seedling into its own cell or small pot; keep the soil moist. Once outdoor soil temperatures reach 65 to 70° F, transplant the seedlings directly into an outdoor garden bed.  

Potting and Repotting

If you're growing dahlias in containers, the bigger the container the better. A good rule of thumb is to use a container that is at least 12 inches deep and wide. If you use containers this large, there should be no need to repot your dahlias during the season. Bring your containers outside in the summer to assure full growth and plentiful blooms, and stake the stems so that they stay upright.

You can also grow compact dahlia varieties, which require less space.

Overwintering

Dahlia tubers need to be dug up and stored for the winter in most zones. To do so, first, select the healthiest plants from your garden, and then wait for the first hard frost and cut the plant back to 4 inches above the ground. Leave the tubers in the ground for one week before digging them up. Dig up each root ball starting at least one foot away from the stem. Carefully remove the tubers (taking care not to damage them) and clean off excessive dirt. Allow the root ball to air dry in a place that is sheltered from frost and out of direct light. At this point, you can either bag and store the whole root ball, or carefully separate the tubers and store them individually. Store your tuber pots, boxes, or bags in a cool, dark, and humid place with temperatures between 40 and 50 F. A basement or root cellar works best; just don't let them freeze.

Common Pests & Plant Diseases

Common pests, like slugs, earwigs, caterpillars, and thrips, adore dahlias. Slugs are especially problematic when the foliage is young and tender. Once the plants mature, slugs are usually not a problem. Some gardeners have trouble with deer, while others claim the deer avoid their dahlias. This may simply depend on the variety of other munchies in your garden. Keep your flowers protected, just in case.

Dahlias are also prone to powdery mildew and other fungal diseases. Keep the foliage as dry as possible and space out your plants to provide good air circulation. If you notice an infection, treat it with neem oil or another natural solution.

How to Get Dahlias to Bloom

Dahlias bloom best when the plant is not flopping to the ground, so be sure to use stakes and twine to keep them up and erect. Fertilize your dahlias every two weeks to allow flowers to proliferate. An organic fertilizer that is high in phosphorous will assure a good flowering rate and strong stems. Be sure to provide ample water for your dahlia bed, especially those containing large varietals, and mulch around the bottom to retain moisture. Lastly, make sure to deadhead your plants as soon as the flowers are spent. removing dead flowers promptly will encourage more blooms.

Common Problems with Dahlias

Stem rot can occur in dahlia beds that have been watered too profusely or in those that have poor drainage and heavy soil. To avoid this, always amend your soil before planting, and never allow for standing water in your garden beds. Dahlias also seem to topple over and wilt during the heat of a midsummer day. This is not necessarily a problem, as it's the plant's way of adapting to stress. If the bed is moist, your dahlias should perk back up once the sun goes down.